U.S. patent application number 12/461210 was filed with the patent office on 2010-02-11 for dcr sense for a cot power converter.
Invention is credited to Chia-Jung Lee, Cheng-De Tseng.
Application Number | 20100033145 12/461210 |
Document ID | / |
Family ID | 41652297 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100033145 |
Kind Code |
A1 |
Tseng; Cheng-De ; et
al. |
February 11, 2010 |
DCR sense for a COT power converter
Abstract
A DCR sense scheme is provided to sense the inductor current of
a COT power converter. The DCR sense is implemented by using the
direct current resistance of the output inductor of the COT power
converter, and thus eliminates the ESR limitations on the type of
output capacitors for stability concern. A quick response mechanism
is further incorporated in the COT power converter to speed up the
transient response of the COT power converter.
Inventors: |
Tseng; Cheng-De; (Tainan
City, TW) ; Lee; Chia-Jung; (Hsinchu City,
TW) |
Correspondence
Address: |
ROSENBERG, KLEIN & LEE
3458 ELLICOTT CENTER DRIVE-SUITE 101
ELLICOTT CITY
MD
21043
US
|
Family ID: |
41652297 |
Appl. No.: |
12/461210 |
Filed: |
August 5, 2009 |
Current U.S.
Class: |
323/282 |
Current CPC
Class: |
H02M 3/156 20130101;
H02M 2001/0009 20130101 |
Class at
Publication: |
323/282 |
International
Class: |
G05F 1/10 20060101
G05F001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 7, 2008 |
TW |
097130139 |
Claims
1. A constant on-time power converter, comprising: an output
capacitor, an output inductor and a switching circuit being
configured such that the switching circuit is controlled by a
control signal to modulate an inductor current flowing through the
output inductor and charging the output capacitor to generate an
output voltage; and a sensing circuit, including: a first resistor
connected between an end of the output inductor and a node; a sense
capacitor connected between the node and the other end of the
output inductor such that the combination thereof with the first
resistor senses the inductor current to generate a feedback signal
to determine the control signal; a second resistor shunt to the
sense capacitor; and a third resistor connected between the node
and a ground terminal such that the combination thereof with the
second and third resistors detects a direct current level of the
output voltage.
2. The power converter of claim 1, further comprising a quick
response mechanism connected to the sensing circuit to trigger a
quick response signal to speed up the transient response of the
power converter.
3. The power converter of claim 2, wherein the quick response
mechanism comprises: a first circuit providing a first signal which
is a function of the output voltage; a second circuit connected to
the first circuit, generating a second signal according to the
first signal and a reference voltage; a third circuit connected to
the second circuit, generating a third signal according to the
second signal and the reference voltage; and a fourth circuit
connected to the third circuit, triggering the quick response
signal according to the third signal and the feedback signal.
4. The power converter of claim 3, wherein the first circuit
comprises a voltage divider dividing the output voltage to generate
the first signal.
5. The power converter of claim 3, wherein the second circuit
comprises a transconductive amplifier connected to the first
circuit, generating the second signal according to the first signal
and the reference voltage.
6. The power converter of claim 3, wherein the third circuit
comprises a reference voltage generator connected to the second
circuit, generating the third signal according to the second signal
and the reference voltage.
7. The power converter of claim 3, wherein the fourth circuit
comprises a comparator connected to the third circuit, triggering
the quick response signal according to the third signal and the
feedback signal.
8. A constant on-time power converter, comprising: an output
capacitor, an output inductor and a switching circuit being
configured such that the switching circuit is controlled by a
control signal to modulate an inductor current flowing through the
output inductor and charging the output capacitor to generate an
output voltage; a sensing circuit connected to the output inductor,
sensing the inductor current to generate a feedback signal; and a
quick response mechanism connected to the sensing circuit,
triggering a quick response signal according to the feedback signal
and the output voltage to speed up the transient response of the
power converter.
9. The power converter of claim 8, wherein the quick response
mechanism comprises: a first circuit providing a first signal which
is a function of the output voltage; a second circuit connected to
the first circuit, generating a second signal according to the
first signal and a reference voltage; a third circuit connected to
the second circuit, generating a third signal according to the
second signal and the reference voltage; and a fourth circuit
connected to the third circuit, triggering the quick response
signal according to the third signal and the feedback signal.
10. The power converter of claim 9, wherein the first circuit
comprises a voltage divider dividing the output voltage to generate
the first signal.
11. The power converter of claim 9, wherein the second circuit
comprises a transconductive amplifier connected to the first
circuit, generating the second signal according to the first signal
and the reference voltage.
12. The power converter of claim 9, wherein the third circuit
comprises a reference voltage generator connected to the second
circuit, generating the third signal according to the second signal
and the reference voltage.
13. The power converter of claim 9, wherein the fourth circuit
comprises a comparator connected to the third circuit, triggering
the quick response signal according to the third signal and the
feedback signal.
14. A control method for a constant on-time power converter
including an output capacitor, an output inductor and a switching
circuit being configured such that the switching circuit is
controlled by a control signal to modulate an inductor current
flowing through the output inductor and charging the output
capacitor to generate an output voltage, the control method
comprising: sensing the inductor current to generate a feedback
signal; and triggering a quick response signal according to the
feedback signal and the output voltage to speed up the transient
response of the power converter.
15. The control method of claim 14, wherein the step of generating
a quick response signal according to the feedback signal and the
output voltage comprises: providing a first signal which is a
function of the output voltage; generating a second signal
according to a difference between the first signal and a reference
voltage; determining a third signal according to the second signal
and the reference voltage; and comparing the third signal with the
feedback signal to trigger the quick response signal.
16. The control method of claim 15, further comprising dividing the
output voltage to generate the first signal.
Description
FIELD OF THE INVENTION
[0001] The present invention is related generally to power
converters and, more particularly, to a constant on-time (COT)
power converter.
BACKGROUND OF THE INVENTION
[0002] In recent years, constant on-time (COT) structure is applied
in wide-input-voltage-range systems generally. However, the
stability of COT structure is always suffered from limitations on
the effective series resistance (ESR) of output capacitors. As
shown in FIG. 1, a COT structure 10 in common use includes a
controller 12 to provide control signals UG and LG to switch the
serially connected power switches SW1 and SW2 in a switching
circuit 14, in order to modulate the output inductor current IL
flowing through an output inductor L1 and charging an output
capacitor COUT to generate an output voltage Vo. The control
signals UG and LG have pulse widths determined by the input voltage
Vin and the output voltage Vo while a constant frequency within the
whole input voltage range. The controller 12 relies on the ESR of
the output capacitor COUT to act as a current sense resistor, so
the ripple of the output voltage Vo provides the pulse width
modulation (PWM) ramp signal. The pulse width of this one shot is
determined by the input voltage Vin and the output voltage Vo to
keep the frequency fairly constant over the input voltage range.
"Double-pulsing" is a way to identify the unstable operation of a
COT structure and occurs due to noise on the output or because the
ESR of the output capacitor COUT is too low that there is not
enough voltage ramp in the output voltage signal. For this reason,
a capacitor with very small ESR, such as a ceramic capacitor, is
not applicable to the output capacitor of a COT structure.
[0003] Therefore, it is desired a solution that can make a COT
structure stable by using ceramic output capacitors and no longer
affects the equivalent value of combination of the ESR of the
output capacitor and capacitive load.
SUMMARY OF THE INVENTION
[0004] An object of the present invention is to provide a direct
current resistor (DCR) sense scheme for a COT power converter.
[0005] Another object of the present invention is to improve load
transient of a COT power converter.
[0006] According to the present invention, a DCR sense scheme is
provided to sense the inductor current of a COT power converter.
The DCR sense is implemented by using the direct current resistance
of the output inductor of the COT power converter to generate a
feedback signal to determine the control signal for controlling the
switching circuit of the COT power converter. The proposed sensing
circuit includes a first resistor connected between an end of the
output inductor and a node, a sense capacitor connected between the
node and the other end of the output inductor, a second resistor
shunt to the sense capacitor, and a third resistor connected
between the node and a ground terminal. The combination of the
sense capacitor and the first resistor senses the inductor current
to generate the feedback signal, and the combination of the first,
second and third resistors detect the DC level of the output
voltage.
[0007] According to the present invention, a COT power converter
does not rely on the ESR of the output capacitor to sense the
inductor current and thus, its stability is not affected by the ESR
of the output capacitor. A quick response mechanism may be
additionally introduced into the COT power converter to trigger a
quick response signal according to the feedback signal and the
output voltage, in order to speed up the transient response of the
COT power converter. The quick response mechanism includes a first
circuit to detect the output voltage to generate a first signal, a
second circuit to generate a second signal according to the first
signal and a reference voltage, a third circuit to generate a third
signal according to the second signal and the reference voltage,
and a fourth circuit to trigger the quick response signal according
to the third signal and the feedback signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] These and other objects, features and advantages of the
present invention will become apparent to those skilled in the art
upon consideration of the following description of the preferred
embodiments of the present invention taken in conjunction with the
accompanying drawings, in which:
[0009] FIG. 1 is a diagram showing a COT structure in common
use;
[0010] FIG. 2 is a diagram of a first embodiment according to the
present invention;
[0011] FIG. 3 is a diagram of a second embodiment according to the
present invention;
[0012] FIG. 4 is a signal flowchart of the quick response mechanism
shown in FIG. 3;
[0013] FIG. 5 is a circuit diagram of an embodiment for the quick
response mechanism shown in FIG. 3; and
[0014] FIG. 6 is a waveform diagram of the COT power converters
shown in FIGS. 2 and 3.
DETAILED DESCRIPTION OF THE INVENTION
[0015] FIG. 2 is a diagram of a first embodiment according to the
present invention, in which a COT power converter 20 includes a
controller 22 to provide control signals UG and LG, and an output
capacitor COUT, an output inductor L1 and a switching circuit 24
being configured such that the switching circuit 24 is controlled
by the control signals UG and LG to modulate the inductor current
IL flowing through the output inductor L1 and charging the output
capacitor COUT to generate an output voltage Vo. In the switching
circuit 24, power switches SW1 and SW2 are connected in series and
switched by the control signals UG and LG, respectively. The direct
current resistance (DCR) of the output inductor L1 is represented
by a resistor RDC. A sensing circuit 26 is connected to the two
ends of the output inductor L1 and senses the inductor current IL
by using the resistor RDC, so as to generate a feedback signal VFB
for the controller 22 to determine the control signals UG and LG.
The sensing circuit 26 includes a resistor R3 connected between an
end of the output inductor L1 and a node A, a sense capacitor C1
connected between the node A and the other end of the output
inductor L1, a resistor R4 shunt to the sense capacitor C1, and a
resistor R5 connected between the node A and a ground terminal GND.
The resistor R3 and the sense capacitor C1 constitute an RC
network, and the resistors R3, R4 and R5 constitute a voltage
divider to detect the DC level of the output voltage Vo.
[0016] As shown in FIG. 2, the DC voltages across the sense
capacitor C1 and the resistor RDC will be the same. Thus, derived
through a voltage-second method, the feedback signal VFB has
variation
.DELTA.VC1=[Vin.times.D.times.(1-D)]/(R3.times.C1.times.Fs),
[Eq-1]
and the output voltage is
Vo = ( R 3 .times. R 4 + R 4 .times. R 5 + R 3 .times. R 5 R 3
.times. R 4 + R 4 .times. R 5 ) .times. VFB - ( R 5 R 3 + R 5 )
.times. Iload .times. RDC , [ Eq - 2 ] ##EQU00001##
where Vin is the input voltage of the switching circuit 24, D is
the duty cycle of the control signal UG, Fs is the operating
frequency of the COT power converter 20, and Iload is the load
current provided by the output Vo of the COT power converter 20.
From the equation Eq-1, the ripple .DELTA.VC1 of the feedback
signal VFB is determined by the resistor R3 and the sense capacitor
C1 and can be used to determine the control signals UG and LG.
Thus, the sensing circuit 26 can extract information about the
inductor current IL. This DCR sense does not rely on the ESR of the
output capacitor COUT to act as a current sense resistor and
provide PWM ramp signal. Thus, the ESR limitations on the type of
output capacitors for stability concern are eliminated. Even if the
output capacitor COUT has no effective series resistance, the COT
power converter 20 can still operate stably. Furthermore, it makes
a small voltage droop by adding the resistor R5 according to the
second term of the equation Eq-2 when the system is operating in
heavy load. In a word, the proposed DCR sense scheme can acquire
PWM ramp signal even if the output capacitors are without ESR
element and make system work under stable situation.
[0017] In order for better transient response, a quick response
mechanism may be additionally introduced into a COT power
converter, as shown in FIG. 3. This power converter 30 has the same
COT structure as that shown in FIG. 2, with a quick response
mechanism 34 connected to the output Vo and the sensing circuit 26
to trigger a quick response signal Sc for a controller 32 to
determine the control signals UG and UL. The quick response
mechanism 34 triggers the quick response signal Sc according to the
feedback signal VFB and the output voltage Vo, in order to improve
load transient of the COT power converter 30.
[0018] FIG. 4 is a signal flowchart of the quick response mechanism
34 shown in FIG. 3, in which a first circuit 3404 detects the
output voltage Vo to generate a voltage Vo_d which is a function of
the output voltage Vo, a second circuit 3402 generates a signal S1
according to the voltage Vo_d and a reference voltage Vref1, a
third circuit 3406 generates a voltage Vref2 according to the
signal S1 and the reference voltage Vref1, and a fourth circuit
3408 triggers the quick response signal Sc according to the voltage
Vref2 and the feedback signal VFB. The circuits 3402, 3404, 3406
and 3408 may be any kind of signal processing circuits. When load
transient occurs, the variation of the output voltage Vo can result
in immediate variation of the voltage Vref2 and consequent
triggering of the quick response signal Sc, thereby speeding up the
transient response.
[0019] FIG. 5 is a circuit diagram of an embodiment for the quick
response mechanism 34 shown in FIG. 3, in which the first circuit
3404 includes a voltage divider 3410 to divide the output voltage
Vo to generate the voltage Vo_d, the second circuit 3402 includes a
transconductive amplifier 3412 to amplify the difference .DELTA.Vg
between the voltage Vo_d and the reference voltage Vref1 to
generate the signal
S1=.DELTA.Vg.times.Gm, [EQ-3]
where Gm is the transconductance of the transconductive amplifier
3412, the third circuit 3406 includes a reference voltage generator
3414 to generate the voltage Vref2 according to the signal S1 and
the reference voltage Vref1, and the fourth circuit 3408 includes a
comparator 3416 to compare the voltage Vref2 with the feedback
signal VFB to trigger the quick response signal Sc. When the COT
power converter 30 operates in steady state, the voltage Vo_d is
equal to the reference voltage Vref1 and therefore, .DELTA.Vg is
zero and quick response is not triggered. In response to load
transient, the output voltage Vo varies and .DELTA.Vg is no longer
zero. As a result, the reference voltage Vref2 is changed to speed
up the transient response.
[0020] FIG. 6 is a waveform diagram of the COT power converters 20
and 30 shown in FIGS. 2 and 3, in which waveform 40 represents the
output voltage Vo of the COT power converter 30, waveform 42
represents the output voltage Vo of the COT power converter 20, and
waveform 44 represents the load current Iload. When load transient
condition occurs at time t1, the load current Iload raises from
Imin to Imax as shown by the waveform 44. The output voltage Vo of
the COT power converter 20 falls down by .DELTA.V1 and then
recovers to its original level as shown by the waveform 42. In
another case, the output voltage Vo of the COT power converter 30
falls down by .DELTA.V2 and then recovers to its original level as
shown by the waveform 40. The COT power converter 30 having the
quick response mechanism 34 can speed up the transient response and
reduce the variation of the output voltage Vo.
[0021] While the present invention has been described in
conjunction with preferred embodiments thereof, it is evident that
many alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, it is intended to embrace
all such alternatives, modifications and variations that fall
within the spirit and scope thereof as set forth in the appended
claims.
* * * * *